A. Technical Field
This invention pertains to methods and apparatus for detecting and distinguishing respiration disorders, especially apneas.
B. Background Art
In 1967, Konno and Mead demonstrated that motion of the respiratory system can be closely approximated by two degrees of freedom. (Konno K., Mead J., Measurement of the Separate Volume Change of Rio Cage and Abdomen During Breathing, J. Appl Physiol. 1967; 22: 407-22) They found that volume change at the mouth (open system) is equal to the sum of the volume change of the rib cage (RC) and abdominal (AB) compartments such that Volume=RC+AB. The occlusion of the mouth or airway (closed system), produced one degree of freedom of motion with any volume change of RC equal and opposite in magnitude to that of AB. This work provided a basis for calibrating devices which monitored changes in RC and AB volume, e.g. linear displacement transducers, magnetometers and the respiratory inductive plethysmograph. In particular, the subject breathed against a valve which was occluded, and then at constant lung volume gently made voluntary movements to shift volume from RC to AB and vice-versa. This "isovolume maneuver" was then used to set the electrical gains of the RC and AB transducers such that the two signals, which are opposite in direction during the maneuver, become equal in amplitude. Once this was accomplished, the subject breathed against an external calibrated volumetric device, such as a spirometer or integrated pneumotachygraph system, whereupon the gain factors of the RC and AB transducers were set to render the SUM (RC+AB) equal to the actual tidal volume reading at the external calibrated volumetric device.
The awareness of the consequences of the isovolume maneuver has also been used to distinguish central apneas from obstructive apneas. These events, which occur during sleep, drowsiness and with narcotic overdose, are well recognized as clinical entites, viz., Cheynes-Stokes Respiration, Obstructive Sleep Apnea Syndrome and Sudden Infant Death Syndrome (SIDS). These disorders can be detected by visual inspection of the waveforms of RC, AB and the SUM of these two compartmental displacments during the apnea. (Tobin M. J., Cohn M. A., Sackner M. A., Breathing Abnormalities During Sleep, Arch. Intern. Med. 1983; 143: 1221-28; Sackner, M. A., Sleep and Arousal Disorders, AMA Guides to Impairment, 2d Ed., 1984; 229-39; Catley D. M., Thornton C., Jordan C., Lehane J. R., Royston D., Jones J. G., Pronounced, Episodic Oxygen Desaturation in the Post-Operative Period: Its Association with Ventilatory Pattern and Analgesic Regimen, Anesthesiology 1985; 63: 20-8; Staats B. A., Bonekat H. W., Harris C. D., Offord K. P., Chest Wall Motion in Sleep Apnea, Am. Rev. Respir. Dis. 1984; 130: 59-63). In central apnea, the respiratory center in the brain fails to command the respiratory muscles to contract and no movements appear on the RC, AB and SUM recordings. Obstructive apneas take place when the tissues of the throat region (upper airway) come into contact with each other so that no air can flow through the mouth and nose despite respiratory muscle efforts. This clinical entity of obstructive apnea is analogous to the isovolume maneuver in that the RC and AB compartments show equal or nearly equal and opposite displacements with the SUM depicting zero or nearly zero displacement. The failure to achieve zero displacement on the SUM signal can result from imperfect initial calibration of the external monitoring device, difference between RC and AB volume-motion coefficients during sleep compared to the waking state when initial calibration was carried out, and thoracic gas compression as the result of vigorous respiratory muscle efforts to overcome the upper airway obstruction. Mixed apneas are a combination of central and obstructive apneas during a single event.
Obstructive apneas can usually be recognized from their appearance on the analog recordings. It should be noted that the diagnosis of obstructive apnea can be made with greater confidence if airflow or volume is measured at the mouth and nostrils, but application of transducers at these orifices is often unsatisfactory because of poor subject cooperation and cumbersome application. Further, in up to 15% of obstructive apneas, displacements on the RC, AB and SUM recordings may be almost imperceptible, and greater confidence in distinguishing central from obstructive apneas is achieved by recording intrapleural pressure swings during the event with an intraesophageal balloon catheter or a surface inductive plethysmographic transducer (such as disclosed in co-pending commonly assigned application Serial No. 789,350) fixed to the skin of the suprasternal fossa. (Tobin M. J., Cohn M. A., Sackner M. A., Breathing Abnormalities During Sleep, Arch. Intern. Med. 1983; 143: 1221-28). However, the esophogeal balloon catheter is an invasive, unpleasant procedure for the subject while the non-invasive surface inductive plethysmograph requires close observation during the recording period due to changes in its electrical gain as a result of changes in neck position.
The detection and differentiation among apneic/hypopneic events depends upon the following. First, the SUM recording from the RC and AB transducers must fall below an empirically determined minimal acceptable volume. Although theoretically the SUM during an apneic event should be equivalent to zero to diagnose apnea, this value cannot be restricted to zero in clinical practice. Even in central apneas, rapid, small changes in volume may occur as a result of the heartbeat which causes gas compression and rarefication within the thorax, and slow drifts may take place because of respiratory gas exchange and shifts of blood volume within the RC and AB during the central apneic event. Furthermore, if the volume to define an apnea is considered as zero, then obstructive apneas will be underestimated because small deflections of the SUM recording as a result of respiratory muscle efforts will occur as mentioned above. Finally, if the minimal acceptable volume is made zero, then minor fluctuations in the waveform within an unimpeded normal breath could be interpreted as breaths by devices, e.g. computers, which generally detect breaths by trough and peak detection. With the potential of small artifactual breaths within the true breath period, the respiratory frequency would be spuriously overestimated. For adults whose normal values for tidal breathing range from 200 to 550 ml (Tobin M. J., Chadha T. S., Jenouri G., Birch S. J., Gazeroglu H. B., Sackner M. A., Breathing Patterns--Normal Subjects, Chest 1983; 84: 202-5), one can arbitrarily denote a minimal acceptable volume as 100 ml or any other appropriate value as empirically determined.
Therefore, an apnea is defined as a period where the SUM tracing falls and stays below the minimal acceptable volume for a defined period of time. Essentially, it can be considered as a prolonged expiration, viz. from the start of expiration to the moment of inspiration. In sleep disorder centers, this time is typically taken as a duration of at least 10 seconds from the point at which zero expiratory flow begins until inspiratory flow begins. This time interval can be shortened or lengthened according to criteria formulated by the observer. A hypopneic event is typically defined as a decrease in tidal volume from a predetermined baseline, e.g., 45% of the mean baseline tidal volume of some other valve chosen by the observer.
When the apnea is detected from the SUM criteria, visual observation of the analog recordings of the RC and AB compartments provide information as to whether the event has a central, obstructive or mixed basis. In patients with Obstructive Sleep Apnea Syndrome, scoring of events can be time-consuming since in severe disease as many as 500 events occur during an 8 hour period. Furthermore, a great deal of paper weighing several pounds is required to record the analog tracings for visual inspection. There are different types of apneas having different causes. Further, hypopneic events may also take place, the latter being a decrease in tidal volume predetermined from a baseline, e.g. 45% of the mean baseline tidal volume or some other arbitrary value chosen by the observer.
In view of the foregoing, it is an object of the present invention to provide an improved method and apparatus for distinguishing among central, obstructive and mixed apneas and hypopneas.
It is a further object to utilize, as part of such improved method and apparatus, conventional devices which monitor two degrees of freedom of motion during respiration, such as a respiratory inductive plethysmograph.